speedometer inaccuracy 1977 Targa

[7783911]

I ve a 77 Targa with 915 Transmission mated to 83 SC engine. My speedo is incorrect and has speed increases so does error...

it reads too HIGH...i havnt figured if thats linear in error or exponential.

Any thoughts on how to tackle this problem...strikes me the pickup sensor must be correct or lower speeds would be off more?

[gjmascoli]

I have a ‘76 all stock and have the same problem. I sent the speedo to NHS for a recalibration, but I still seem to have the same error.

[dap930]

You could install a speedometer calibrator electronic module that intercepts the pulse signal from the reed switch coming from the transmission and alters the pulse rate going into the speedometer to correct the speed reading. It can be installed behind the dash and you can piggy back 12vdc and ground from the speedometer connections. There are two red/brn colored wires that come from the transmission reed switch to the speedometer. One goes to ground and the other feeds the speedometer circuitry. The wire going the speedometer circuitry is the wire that the speedometer calibrator will hook into. You can use the speed reading from a Garmin, etc. to check and adjust the calibrator circuit. There are 3 or 4 speedometer calibrators available on the market. Some are more expensive than others. I know there is at least one thread on this forum that deals with this subject if you do a search. I installed a SpeedoDRD Speedo Calibrator on my turbo, but I had to make a small adapter circuit board to go with it to make it work (I can provide the info if needed). Other brands do not need this. I installed the calibrator in the tunnel where the shift coupler is because it is easier to access for adjustments, but I had to run a 12vdc wire in the tunnel to power it. Also, I found that for my speedometer at least, the error was not linear. So the speedometer reads a little high at low speeds, but it is dead on at 60 - 70 MPH. I ended up lowering the pulse rate by 5.8% for this particular unit.

[Jack Stands]

Is the 77 a mechanical or electronic speedometer? I found using the GPS speedometer on my iPhone is probably the easiest way to be accurate as the VDOs do tend to be overly optimistic on your actual speed (good for bragging rights I guess).

[fanaudical]

If you're adventurous and have the electronic speedo, there is an internal potentiometer you can adjust for speedometer calibration. I did this when replacing my broken odometer gears (my speedo used to read high as well). My process:

- Drove on a long straight stretch on the highway with my GPS receiver; noted speedometer reading at 60mph indicated on the GPS.

- Took the speedometer out of its housing.

- Improvised a speed signal with an old set of points, a hex shaft, and my drill press.

- Adjusted the speed of the drill press so that the speedo read what I recorded in the first step.

- Adjusted the speedo potentiometer until it read 60mph.

I have checked it several times since at various speeds against my GPS receiver and those silly radar speed carts - it's still right on.